A thermal transfer recording medium, so-called ink ribbon, on which a thermal fusion wax layer and a thermal sublimation transfer layer are formed makes direct contact with a thermal head; therefore, the medium requires heat resistance and a sliding ability, and for such, a heat-resistant sliding layer is usually formed on the backside of a support (a base film).
Many studies have been made on the heat-resistant sliding layer composed of silicone-based materials. Such silicone-based materials include heat-resistant resins such as silicone resin, epoxy resin or the like (Japanese patent application laid-open publication No. 7,467/80, etc.), silicone wax (Japanese patent application laid-open publication No. 137,693/85, etc.), silicone oil (Japanese patent application laid-open publication No. 148,697/84, Japanese patent application laid-open publication No. 273,887/87, etc.).
However, as speed of thermal transfer printers increases and the shape of a thermal head changes, various problems are posed. For example, residue is formed when the thermal head bears against the heat-resistant sliding layer, and the layer is allowed to melt by the heat of the thermal head, resulting in its sticking. Residue forming at the heat-resistant sliding layer is such a phenomenon that the layer is ground by contact with the thermal head and resulting residue sticks to the thermal head, depending on the adhesion between the base film and heat-resistant sliding layer, the sliding ability, hardness of the heat-resistant sliding layer, and the like. If these characteristics are not well-balanced, the entire layer falls off in the worst cases.
This phenomenon leads to the inability of print at the part where residue remains between the heat-resistant sliding layer and the thermal head resulting in poor heat conductivity from the thermal head to an ink layer. As a result, there is a tendency that printed letters and numbers become illegible and a record can also be misread, as in the case of printed bar codes or the like.
Moreover, particularly in the case where silicone oil is used, the silicon oil is transferred to the ink layer and the recording quality becomes degraded if the ink ribbon is stored with the ribbon rolled up tightly.
In order to solve these problems, there have been attempts to modify the silicone oil and resin, and it has been reported that silicone-(meth)acrylic graft polymer made from the combination of silicone macromer and (meth)acrylic polymer showed good results (for example, Japanese patent application laid-open publication No. 143,195/86, Japanese patent application laid-open publication No. 1,575/87, Japanese patent application laid-open publication No. 30,082/87, Japanese patent application laid-open publication No. 214,475/89, Japanese patent application laid-open publication No. 274,596/90, and the like.)
In the case where the aforesaid silicone-(meth)acrylic graft polymer is used for the heat-resistant sliding layer, the (meth)acrylic component is oriented on a side of a support (a base film), and the silicone is oriented on the other side, or the side of the heat-resistant sliding layer. Therefore, both strong adhesion to the support and good sliding ability are expected to be achieved.
However, as a result of study and investigation made by the present inventors, it has been formed that further improvement was necessary since the residue formation still remained and the amount of residue increased with high-speed printing though the sliding characteristics were slightly improved by using the aforesaid silicone-(meth)acrylic graft polymer.